"I love working with him so much. I just want to do that forever, so who knows?" she said.

The pair's reunion on Ruben Fleischer's Prohibition-era drama was a very different affair. In the film, Emma plays actress Grace Faraday who is dating Mafia gangster Mickey Cohen (Sean Penn), while having a brief romance with LAPD detective sergeant Jerry Wooters (Ryan).

"It was hard because we were so jokey with each other, so much of the time it was pretty tough not to crack jokes. But it was really cool to do something different with him," the actress added.

Emma admitted she was in awe of co-star Sean.

"He was not intimidating. I really admire him as an actor, so I was more excited than anything or nervous to work with him. The best thing about him is the fact he can flip in and out of a character, he doesn't have to stay in character in between takes. He can just turn it on and off."

Josh Brolin, Giovanni Ribisi and Michael Pena also star in Gangster Squad, which opens in cinemas on January 10.